make remoteKeyToRemoteName safer

If it's passed a ConfigKey such as annex.version, avoid returning
an empty remote name and return Nothing instead. Also, foo.bar.baz is
not treated as a remote named "bar".

Remote/Helper/Special.hs

@@ -61,7 +61,7 @@ import qualified Data.Map as M
 findSpecialRemotes :: String -> Annex [Git.Repo]
 findSpecialRemotes s = do
 	m <- fromRepo Git.config
-	liftIO $ mapM construct $ remotepairs m
+	liftIO $ catMaybes <$> mapM construct (remotepairs m)
   where
 	remotepairs = M.toList . M.filterWithKey match
 	construct (k,_) = Git.Construct.remoteNamedFromKey k